The present invention relates in general to a device for centering clamping of workpieces in the field of high-precision optics, the watch and clock making industry and the semiconductor industry, where workpieces have initially to be clamped in a centred manner and subsequently machined at the edge and/or scanned. In particular, the invention relates to a device for centering clamping of optical lenses, by means of which it is possible to clamp a lens in centred manner for edge machining thereof by the so-called bell clamp method.
Lenses for objectives and the like are xe2x80x9ccentredxe2x80x9d after machining of the optical surfaces, so that the optical axis, the position of which is distinguished by the straight line through the two centres of curvature, also passes through the geometric centre of the lens. To this end, the lens is initially aligned and clamped between two aligned centering spindles in such a way that the two centres of curvature of the lens coincide with the common axis of rotation of the centering spindles. The edge of the lens is then machined in a defined relationship to the optical axis of the lens, as is necessary later for mounting the lens in a holder. During this process, the edge, depending on the material of which the lens is made (glass or plastics), is provided with a defined geometry, both in plan view onto the lens (circumferential contour of the lens) and viewed in radial section (contour of the edge, for instance linear construction or construction with bevel(s)), by machining with geometrically non-specific or specific cutting edges. Thus, during so-called centering of optical lenses, it is necessary to distinguish between the actual aligning and clamping process, with which the present invention is primarily concerned, and the subsequent machining of the lens edge.
A general overview of current centering technology practice is provided in this context by the article xe2x80x9cWas leisten moderne Zentriermaschinen?xe2x80x9d by Dipl.-Ing. (FH) Michael Leitz, published in xe2x80x9cJahrbuch fxc3xcr Optik und Feinmechanik 1999xe2x80x9d (Ed.: Dr.-Ing. Wolf-Dieter Prenzel; Fachverlag Schiele and Schxc3x6n GmbH, Berlin; ISBN 3 7949 0634 9), pages 161 to 175.
The above-mentioned bell clamp process is understood to mean an aligning and clamping process, in which the lens and its optical axis are aligned and clamped automatically relative to the vertically extending axis of rotation of the centering spindles between cup-shaped bell clamps provided on the centering spindles. To this end, the lens is positioned on the bell clamp of the lower centering spindle and the bell clamp of the upper centering spindle is displaced in the axial direction relative to the lower bell clamp until the upper bell clamp also lies with slight pressure against the lens. The lens is then displaced automatically in the transverse direction, as a result of the curvature of its optical surfaces, optionally with the addition of a suitable lubricant and/or rotation of the centering spindles, wherein the bell clamps move closer together. The transverse movement of the lens relative to the bell clamps and the axial relative movement of the bell clamps ends when the lens has assumed a position between the bell clamps which allows the minimum spacing of the bell clamps under the given geometric conditions. The lens, which then has its optical axis aligned relative to the axis of rotation of the centering spindles, is then clamped firmly between the bell clamps by increasing the clamping force and may be machined at the edge. The above-described bell clamp method reaches its limits in the case of lenses with only slightly curved optical surfaces. Below a certain value of the angle, also designated as the centering angle, formed between a tangent to the edge of the one optical surface of the lens at the clamping point (= contact point of the bell clamp) and a tangent to the edge of the other optical surface at the clamping point when viewed in radial section, self-locking arises, which prevents transverse movement of the lens relative to the bell clamps.
A device for centering clamping of optical lenses for edge machining thereof, which is designed to operate according to the above-described bell clamp method, has, inter alia, to the fulfil the following requirements. On the one hand, the axial alignment of the two centering spindles may deviate from one another by at most a few thousandths of a millimeter and must also be maintained during the entire clamping movement, i.e. the relative axial movement of the centering spindles. On the other hand, the clamping movement has to proceed as smoothly or non-jerkily as possible. This applies in particular to the moment at which the lens is caught between the two bell clamps. At this point, jerky movements must be avoided, so that the lens may slip into its optical axis and undergo automatic alignment without the risk of mechanical damage.
The prior art is not short of proposals for constructing a device for centering clamping of optical lenses. Thus, a machine for centering edge grinding and bevelling of optical lenses is known from DE 37 44 115 C2, DE 37 44 116 C2 and DE 37 44 118 C2 held by the applicant as signee, which machine comprises in a machine frame two axially aligned centering spindles, which carry bell clamps at their mutually facing ends. The lens may be chucked between the bell clamps for machining purposes by means of a clamping device acting on the axially displaceably guided lower centering spindle. Each of the centering spindles is here arranged in a quill and supported therein by supporting bearings.
In this prior art, the quill of the lower centering spindle is guided in a plurality of air bearings, which are formed in a thin-walled guide sleeve held in the machine frame. The guide sleeve tightly surrounds the quill and is in turn surrounded by a cavity formed in the machine frame and pressurisable by a pressure medium. The quill air bearing system is designed to provide very small forces for aligning the lens, sensitive adjustability of these forces, jerk-free advance of the lower centering spindle and high axial alignment precision of the centering spindles, whereby damage of the optical surfaces of the lens during alignment thereof should be avoided. Once the lens has reached its precise alignment position, the cavity surrounding the guide sleeve is pressurised with high pressure, such that the quill of the lower centering spindle is clamped in its respective position and the disadvantage immanent in the air bearings during machining, namely their low rigidity, which is insufficient to produce a good working result, is countered. However, a disadvantage of this prior art is in particular that smooth-running but nonetheless centred guidance of the quill of the lower centering spindle during alignment of the lens is bought at great cost with regard to apparatus and control systems.
The same is true of the clamping apparatus which, according to this prior art, comprises a plate-like yoke arranged beneath the quill of the lower centering spindle, in which yoke a diaphragm piston/cylinder unit is arranged centrically relative to the lower centering spindle, which piston/cylinder unit acts on the lower end of the lower centering spindle, and to which yoke a double-acting pressure cylinder with a short-stroke and a long-stroke piston is attached on each side of the centering spindle axis.
While the pressure cylinders here generate the stroke, until the long-stroke piston lies against the short-stroke piston, which is required to bring the bell clamps close enough together to leave a slight gap between the upper bell clamp and the lens lying on the lower bell clamp, the diaphragm piston/cylinder unit serves as a precision stroke means for the lens alignment process, by means of which the required clamping force for aligning the lens may be sensitively established.
In addition, DE 31 39 873 A1 discloses a centering device for a machine for edge grinding and bevelling optical lenses, having two centering spindles arranged in aligned manner one above the other, the lower of which is stationary while the upper one is mounted in axially displaceable manner and is under axial pressure loading in order to hold the optical lens. Axial displacement of the upper centering spindle into the adjusting position here proceeds, as does securing of the upper centering spindle in the adjusting position, by means of two lifting cylinders connected in parallel. The two lifting cylinders act simultaneously on a supporting plate, to which end the pistons of the lifting cylinders are brought into firm connection with the supporting plate by pressure bars. The two lifting cylinders may be caused to perform a lifting or lowering movement via a control means, in which movement there participate the supporting plate and the upper centering spindle, brought into axial driving connection with the supporting plate.
In addition to the complex control means required here to ensure synchronous lifting cylinder strokes, a disadvantage of this prior art is that stick-slip effects may arise in the lifting cylinders, which do not allow the sensitive, jerk-free advance of the upper centering spindle necessary for automatic aligning of the lens.
Finally, a device for centering optical lenses is described in DE 198 25 922 A1 which has two centering spindles arranged in aligned manner one above the other, which carry bell clamps for the lens at their mutually facing ends. While the lower centering spindle is here connected firmly to a machine frame, the upper centering spindle is guided in a guide cylinder by means of annular plain bearings, not described in any more detail, at the circumference of the centering spindle, such that it may perform axial movement. The guide cylinder connected firmly with the machine frame comprises a recess through which a gearwheel extends, which engages with external teeth on the upper centering spindle and effects axial feed thereof during rotation. The gearwheel is seated for rotation on a shaft mounted in the machine frame, which shaft is actively connected with an electric motor via a belt drive and with a compressed air cylinder via a lever. Finally, a manual lever is also attached to the shaft. With the aid of the electric motor, it is intended that the upper centering spindle be moved up and down in the axial direction inter alia for automatic centering of lenses. If centering is to be performed by manual operation, the upper centering spindle may be moved up and down by means of the manual lever against the force of the compressed air cylinder, which in this case generates the feed force.
A disadvantage of this prior art is that, on the one hand, the unilateral engagement of the gearwheel with the external teeth on the upper centering spindle produces a moment there which tends to tilt the upper centering spindle relative to the guide cylinder in the play-affected guide provided by the guide cylinder, which may result in a not inconsiderable alignment error between upper and lower centering spindles. On the other hand, the feed movement of the upper centering spindle may cause jerking as a result of the play between the teeth of the gearwheel and the external teeth on the upper centering spindle. To this may be added the fact that, owing to the stick-slip effects in the compressed air cylinder coupled compulsorily with the gear shaft via the lever, a jerk-free feed movement of the upper centering spindle cannot be guaranteed. Consequently, this centering device does not seem suitable for clamping in particular small lenses automatically by the bell clamp method without the optical surfaces of the lens being damaged.
Taking as basis the prior art according for example to DE 31 39 873 A1, the object of the invention is to provide a simply constructed device for centering clamping of workpieces, in particular optical lenses, for machining of the edges thereof, which allows a smooth and sensitively adjustable clamping movement with precise axial alignment of the centering spindles, such that it also allows in particular automatic clamping of small lenses by the bell clamp method, without damage to the optical surfaces of the lens.
This object is achieved by the features indicated in the independent claims. Advantageous or convenient modifications of the invention constitute the subject matter of the dependent claims.
According to a first aspect of the present invention, a device for centering clamping of workpieces, in particular optical lenses, for machining of the edges thereof, having two aligned centering spindles arranged one above the other, which spindles are each constructed at their mutually facing ends to accommodate a bell clamp, wherein at least one centering spindle is guided axially in a centering spindle guide and may be moved in the axial direction relative to the other centering spindle by means of a lifting apparatus, in order to align and chuck the workpiece, has a swivellable rocking lever, to one end of which there is coupled the axially mobile centering spindle and to the other end of which there is coupled at least one counterweight, in order to produce a moment at the rocking lever which counteracts the moment produced by the axially mobile centering spindle.
In contrast to the above-described prior art, the lifting apparatus here does not have to lift or hold the entire weight of the axially mobile centering spindle, but rather has only to oppose a slight force resulting from the moment arising at the rocking lever, which force preferably acts in the direction of the other centering spindle, which allows a very sensitive contacting or clamping movement of the axially mobile centering spindle in the direction of a lens to be clamped with very slight and readily apportionable forces, such that in particular very small lenses may be aligned and clamped automatically by the bell clamp method without the risk of damage to their optical surfaces or breaking of the lens. The slight force resulting from the moment arising at the rocking lever may be simply adjusted with respect to sign and amount in accordance with the respective requirements by a suitable choice of the lever arm ratio at the rocking lever or the mass of the counterweight.
The rocking lever may in principle also be arranged beneath the centering spindles. However, a design is preferred, according to which the rocking lever is arranged above the axially mobile upper centering spindle. This arrangement has, inter alia, the advantage that the rocking lever mechanism is less easily soiled and thus its smooth running remains guaranteed, in particular when the device according to the invention is a component of a machine for edge machining workpieces, in particular for centering edge-grinding and bevelling of optical lenses, which additionally comprises at least one driven tool spindle for a tool which may optionally be brought into engagement with the workpiece.
The rocking lever may have a forked portion, on which rollers are rotatably mounted, which engage with a drive flange attached to the axially mobile centering spindle. This ensures that, when the rocking lever swivels, no forces acting perpendicularly to the centering spindle axis are introduced into the axially mobile centering spindle, which could impair the axial alignment of the centering spindles and/or the smooth running of the centering spindle guide.
Although it is in principle possible for the lifting apparatus to be arranged on the centering spindle side of the rocking lever, an arrangement is preferred, according to which the lifting apparatus acts on the end of the rocking lever to which the counterweight is coupled. On the one hand, this allows the construction of the device according to the invention to be very compact. On the other hand, the lifting apparatus may thus be arranged spatially separately from the centering spindles when the device according to the invention is used in an edge machining machine, whereby soiling of the lifting apparatus, which might impair the smooth running of the lifting apparatus, is prevented.
The lifting apparatus can comprise a limit stop movable in the axial direction, which limit stop serves to absorb the moment arising at the rocking lever or forces resulting therefrom. In this embodiment of the device according to the invention, the axially mobile centering spindle, when performing its contacting or clamping movement, initially follows the axial movement of the lifting apparatus, with the rocking lever lying against the limit stop. When the bell clamp of the axially mobile centering spindle lies against the lens to be aligned or clamped with a slight force definedly adjustable by a suitable choice of the lever arm ratio at the rocking lever or the mass of the counterweight, the rocking lever then moves out of engagement with the limit stop of the lifting apparatus, such that the alignment process of the lens or the movement thereof perpendicular to the centering spindle axis may advantageously proceed uncoupled from the lifting apparatus.
The lifting apparatus may have a spring mechanism, by means of which a defined, additional force may be applied to the axially mobile centering spindle in the direction of the other centering spindle. Thus, an apportionably higher force may be applied via the spring mechanism to the workpiece to be aligned or clamped, which allows the workpiece to be chucked so firmly, for instance for edge machining thereof, that transverse forces acting on the workpiece cannot displace the latter out of its axially aligned position. The clamping force profile may here be readily adjusted or modified in accordance with the respective requirements by a suitable choice of springs with a specific spring characteristic. The spring mechanism may comprise one or more tension springs. However, it is preferable for the spring mechanism to comprise at least one compression spring.
According to a second aspect of the present invention, in the case of a device for centering clamping of workpieces, in particular optical lenses, for edge machining thereof, having two aligned centering spindles arranged one above the other, which spindles are each constructed at their mutually facing ends to accommodate a bell clamp, wherein at least one centering spindle is guided axially in a centering spindle guide and may be moved in the axial direction relative to the other centering spindle by means of a lifting apparatus, in order to align and chuck the workpiece, provision is made for the lifting apparatus to comprise a ball screw drivable by means of an electric motor, which ball screw serves to move one centering spindle relative to the other centering spindle in the axial direction, preferably under CNC control, in order to align the workpiece and also chuck it.
With the aid of only one driven ball screw, the entire movement process of the axially mobile centering spindle, i.e. the feed, alignment and clamping movement thereof, may thus be performed smoothly and with sensitively apportionable forces. Jerky relative movement of the centering spindles, as may occur in the above-described prior art for instance as a result of stick-slip effects in the lifting mechanism, is here ruled out by the ball screw, which in particular allows automatic, damage-free clamping of very small lenses by the ball clamp method.
The ball screw conveniently comprises a rotatably mounted roller ball spindle connected for drive with the electric motor, which spindle engages with a nut which is connected non-rotatably with a linearly guided clamping saddle. In an advantageously simple embodiment, at least one stay bolt is attached to the clamping saddle, at whose end remote from the clamping saddle there is attached the limit stop serving to absorb the moment arising at the rocking lever or forces resulting therefrom. In addition, the same stay bolt may also pass through and thus mount the above-mentioned compression spring of the spring mechanism, wherein a plate is guided longitudinally displaceably on the stay bolt on the side of the compression spring remote from the clamping saddle, which plate may optionally be brought into engagement with the rocking lever by means of the ball screw, in order to effect the above-described increase in clamping force.
An additional lever mechanism may be provided, by means of which the axially mobile centering spindle may be moved away manually from the centering spindle. This is particularly advantageous when lenses with very unfavourable centering angles are clamped between the centering spindles, where alignment relative to the centering spindle axis is possible only by hand. The additional lever mechanism conveniently acts on the rocking lever or the counterweight.
According to a third aspect of the present invention, in the case of a device for centering clamping of workpieces, in particular optical lenses, for edge machining thereof, having two aligned centering spindles arranged one above the other, which spindles are each constructed at their mutually facing ends to accommodate a bell clamp, wherein at least one centering spindle is guided axially in a centering spindle guide and may be moved in the axial direction relative to the other centering spindle by means of a lifting apparatus, in order to align and chuck the workpiece, provision is made for the centering spindle guide to comprise at least two linear guide units, which are arranged on each side of the axially mobile centering spindle in substantially play-free manner between the axially mobile centering spindle and a box-type structure surrounding it.
Thus, a smooth-running but nonetheless highly rigid centering spindle guide is provided for the device for centering clamping of workpieces, which ensures high precision of axial alignment for the centering spindles even under the action of external forces (e.g. machining forces in the event of grinding of the edge of a chucked lens). Because the linear guide units are arranged on each side of the axially mobile centering spindle, the centering spindle guide advantageously additionally exhibits thermally invariable behaviour in which thermal expansion is mutually compensated or supported relative to the box-type structure and thus does not impair the axial alignment of the centering spindles. Furthermore, the very compactly constructed box-type structure here replaces in a simple, assembly-friendly manner a complex cast gantry for the axially mobile centering spindle, which was necessary in the prior art, and additionally ensures boxing-in of the centering spindle guide, such that the latter is not susceptible to the effects of external dirt.
The axially mobile centering spindle can comprise a spindle housing and a spindle shaft mounted rotatably therein, wherein each linear guide unit has a carriage attached conveniently to the spindle housing, which carriage is guided on a respectively associated guide rail attached to the box-type structure.
Each carriage may be equipped with a plurality of ball chains, which run in respectively associated longitudinal channels in the corresponding guide rail. Such compact guides are commercially available bought-in components and are distinguished by their smooth running or lack of jerkiness, rigidity and low wear and maintenance.
Because the spindle housing may comprise, at least in part, a substantially rectangular external cross section, wherein a stop strip, extending parallel to the centering spindle axis, for the corresponding carriage is constructed on each opposing side face of the spindle housing, the linear guide units are easily aligned relative to the centering spindle axis.
In an advantageously simple embodiment, the box-type structure has four side walls preferably screwed together, which side walls define a cross-sectionally substantially rectangular cavity, in which the centering spindle guide is arranged. Measures can be provided which ensure in a simple manner, in the case of such an embodiment of the box-type structure, inter alia precise alignment of the guide rails relative to the centering spindle axis and lack of play in the centering spindle guide. Accordingly, the guide rails of the centering spindle guide are attached to opposing side walls of the box-type structure, wherein one of these side walls comprises a stop surface extending parallel to the centering spindle axis for the corresponding guide rail. The side walls themselves bearing the guide rails of the centering spindle guide are arranged between the other two side walls, whereby the box-type structure is very compact, wherein the other side walls in each case comprise only one stop strip extending parallel to the centering spindle axis for the same side wall bearing the corresponding guide rail. A fixed bearing is thus virtually produced both for one of the guide rails and for one of the guide rail side walls by the stop surface on the one guide rail side wall or the stop strips on the other side walls, while the other guide rail or the other guide rail side wall undergoes virtually movable bearing attachment without a limit stop, by means of which tolerances may be compensated, such that the centering spindle guide may be mounted in play-free manner. Angular offset between the axially mobile centering spindle and the other centering spindle is then ruled out in that the box-type structure is surface-ground on a bearing surface perpendicular to the centering spindle axis after assembly of the side walls, which bearing surface rests on a machine frame, which supports the other centering spindle.
The centering spindle guide can be conveniently equipped with a preferably contactless measuring system for a CNC control system, which comprises a slider attached to the axially mobile centering spindle together with a detection unit for the slider, fixed to the box-type structure. Thus, the contacting or clamping movement of the axially mobile centering spindle may be directly detected and sensitively controlled on the basis of the detected values. This measuring system may in principle be arranged inside the box-type structure. However, a more compact design is preferred, according to which the slider is attached to one of the carriages of the linear guide units with at least one stay bolt, which passes through an opening in a side wall of the box-type structure, on which the detection unit is arranged.
In an advantageous embodiment of the device according to the invention, one of the centering spindles is stationary in the axial direction and may be driven by means of a preferably CNC-controlled rotary actuator arranged concentrically to the centering spindle axis. The concentric arrangement of the rotary actuator has the advantage, on the one hand, that the device is very compact construction. On the other hand, high axial alignment precision of the centering spindles is ensured because, during drive of the stationary centering spindle, unilaterally acting transverse forces are no longer introduced thereinto from outside, as is the case with the gearing or belt drives according to the prior art. This rotary actuator can also conveniently drive the axially mobile centering spindle via a first gear pair, a vertical shaft and a second gear pair, optionally with the interposition of an apparatus for evening out the rotary movements of the centering spindles.
Finally, in the case of a machine for edge machining of workpieces with a clamping device according to the invention and at least one driven tool spindle, the tool spindle can extend parallel to the centering spindle axis to be offset angularly about the centering spindle axis relative to the linear guide units of the centering spindle guide. In this way, it is on the one hand ensured that the centering spindle guide does not spatially hamper the tool spindle or the guide thereof. On the other hand, it is thus possible to minimise the distance between centering spindle axis and tool spindle axis, such that for example lenses with very small diameters clamped between the centering spindles may also be machined at the edge, for which purpose, for instance, grinding wheels with small external diameters may be used. The use of small grinding wheels is desirable in any case for reasons of cost and weight.
In conclusion, it should be stated that, according to the invention, a device for centering clamping of workpieces is provided which, owing to the smooth running of centering spindle guide and lifting apparatus, the constantly guaranteed precise axial alignment of the centering spindles and the sensitively adjustable forces during alignment of the workpiece, allows for the first time even very small lenses for instance for endoscopic applications or the like possibly with unfavourable centering angles to be automatically aligned or clamped by the bell clamp method, without the lenses being damaged.